Parasitic helminth infections in animals, including humans, are typically treated by chemical drugs. One disadvantage with chemical drugs is that they must be administered often. For example, dogs susceptible to heartworm are typically treated monthly. Repeated administration of drugs, however, often leads to the development of resistant helminth strains that no longer respond to treatment. Furthermore, many of the chemical drugs cause harmful side effects in the animals being treated, and as larger doses become required due to the build up of resistance, the side effects become even greater. Moreover, a number of drugs only treat symptoms of a parasitic disease but are unable to prevent infection by the parasitic helminth.
An alternative method to prevent parasitic helminth infection includes administering a vaccine against a parasitic helminth. Although many investigators have tried to develop vaccines based on specific antigens, it is well understood that the ability of an antigen to stimulate antibody production does not necessarily correlate with the ability of the antigen to stimulate an immune response capable of protecting an animal from infection, particularly in the case of parasitic helminths. Although a number of prominent antigens have been identified in several parasitic helminths, there is yet to be a commercially available vaccine developed for any parasitic helminth.
As an example of the complexity of parasitic helminths, the life cycle of D. immitis, the helminth that causes heartworm disease, includes a variety of life forms, each of which presents different targets, and challenges, for immunization. In a mosquito, D. immitis microfilariae go through two larval stages (L1 and L2) and become mature third stage larvae (L3), which can then be transmitted back to the dog when the mosquito takes a blood meal. In a dog, the L3 molt to the fourth larval stage (L4), and subsequently to the fifth stage, or immature adults. The immature adults migrate to the heart and pulmonary arteries, where they mature to adult heartworms. Adult heartworms are quite large and preferentially inhabit the heart and pulmonary arteries of an animal. Sexually mature adults, after mating, produce microfilariae which traverse capillary beds and circulate in the vascular system of the dog.
In particular, heartworm disease is a major problem in dogs, which typically do not develop immunity, even upon infection (i.e., dogs can become reinfected even after being cured by chemotherapy). In addition, heartworm disease is becoming increasingly widespread in other companion animals, such as cats and ferrets. D. immitis has also been reported to infect humans. There remains a need to identify an efficacious composition that protects animals and humans against diseases caused by parasitic helminths, such as heartworm disease. Preferably, such a composition also protects animals from infection by such helminths.
The cuticle is an important part of the nematode's exoskeleton and protects the animal from the environment under a variety of conditions. In addition, it also mediates the metabolic interaction of the animal with its environment and, in parasitic nematodes, the interaction with the host and its immune system. The nematode cuticle is a complex extracellular structure that is secreted by an underlying syncytium of hypodermal cells. Recent studies have demonstrated that the cuticle of parasitic nematodes is a dynamic structure with important absorptive, secretory, and enzymatic activities, and not merely an inert protective covering as was once believed. See, for example, Lustigman, S. 1993, Parasitology Today, 9:8, 294-297. In addition, immunological studies have shown the central importance of cuticular antigens as targets for protective immune responses to parasitic nematodes. In spite of the wide recognition of the importance of the cuticle in the nematode physiology and its potential role as a target for immunoprophylaxis, relatively little is known about the biology of the cuticle of filarial parasites. Though a number of collagen genes have been characterized in filarial parasites, very little is known about the non-collagenous cuticular proteins, including cuticlin, in filarial parasites. Prior studies in C. elegans have shown that cuticlin genes are developmentally regulated and that the message for one of the C. elegans cuticlins, cut-1, is up-regulated during larval molting. Antibodies raised against a cuticlin of Ascaris cross-react with the epicuticular structures of filarial parasites indicating that components of cuticlin are immunogenic. Since cuticlin proteins are highly conserved among nematodes, but not among other organisms, they could be an important target for protective immunity to parasitic helminths.